Method of porcelain enameling metal foils



United States Patent Ofilice 3,476,584 Patented Nov. 4, 1969 3,476,584 METHOD OF PORCELAIN ENAMELING METAL FOILS Ronald M. Randklcv, White Bear Lake, and Clarence Raymond Humpllries, Stillwater, Minn, assignors to Minnesota Mining and Manufacturing Company, St. Paul, lVIinm, a corporation of Delaware No Drawing. Filed Mar. 21, 1966, Ser. No. 535,711 Int. Cl. B44d 1/34, 1/46 US. Cl. 117-66 4 Claims ABSTRACT OF THE DISCLOSURE Thin metal foil is coated over one surface with an aqueous slip of porcelain frit and free silica, dried and partially sintered, wound into coils for economical further firing to provide an acid-resistant coating and then uncoiled.

This invention relates to the porcelain enameling of metal foils, and more specifically to a method of firing the coated foil.

Porcelain enamel coatings on metal substrates are conventionally produced by first applying to the clean metal surface a coating of water-suspended enamel-forming composition or slip, which is then dried and fired. The dry residue prior to firing is extremely fragile and is easily abraded or flaked from the metal surface. Even with rigid metal articles such as kitchen ware, precautions must be taken to avoid loss of the coating prior to and during firing. The problem is compounded in the case of thin foils because of the flexibility of the foil.

One method which has previously been suggested for firing enamel-coated metal foil is illustrated in US. Patent No. 2,827,393 wherein the cleaned and coated foil is supported on a conveyor belt while being continuously advanced beneath a series of infra-red lamps. The enameled foil is subsequently smoothed between heated pressure rollers. Somewhat analogously, U.S. Patent No. 2,676,899 describes the enameling of wire mesh in a continous process wherein the slip-coated self-supported mesh is drawn into and through a gas-fired furnace where the slip is dried and fired. Without adequate pre-drying, such techniques are found to produce blistering and pinholing in porcelain coatings. The relatively long time required for drying and firing makes it necessary either to use ovens of excessive length or to operate at undesirably low speeds. The problem is further complicated when light-weight easily distorted metal foils of extreme thinness and relatively low tensile strength are involved, such for example as the thin soft aluminum foil employed in copending United States application Ser. No. 482,838 filed Aug. 26, 196 5 and now abandoned, the disclosure of which is incorporated herein by reference.

It has now been found that slip-coated thin metal foils, when first completely dried and briefly heated to provide preliminary partial sintering of the glassy frit component of the slip, may then be wound into coil or roll form and subjected to final firing at a higher temperature without loss of coating and with formation of a dense porcelain enamel finish of outstanding uniformity.

Enamel coatings on aluminum, having particularly good chemical and weather resistance and based on compositions as described in the above-mentioned application Ser. No. 482,838, are in particular produced both economically and with improved properties by the method of the present invention. These composition combine a low silica content glassy frit, capable of being fused at a temperature below the softening or melting point of aluminum, with an amount of free silica in finely divided particulate form sufiicient to raise the fusion temperature of the pre-fused and cooled mix well above the melting point of the aluminum. Prolonged heating at a temperature somewhat below the softening point of the foil is found to produce a porcelain enamel coating having an excellent bond to the metal substrate as well as exceptional chemical resistance. Firing is accomplished economically and effectively by first placing the coated material in roll or coil form, where as in continuous strip firing the time required would make the process impossible or at best commercially impractical. Smooth uniform coating is costly and is difficult to achieve at very slow rates of travel of the web; ovens of excessive length offer problems in the accurate control of temperature; extend d lengths of foil are ditficult to handle without wrinkling, sagging, or breaking.

The degree of prelminary sintering required to enable the coated foil to be wound up in rolls or coils is surprisingly slight, and may be achieved by brief heating to a moderately high temperature or by longer heating at somewhat lower temperature; but substantially more than simple drying is involved, as may be shown by the following simple test. A drop of water is placed on the surface of the coating and its action observed. On both the dry and the dried and sintered coating the drop is rapidly absorbed; but gentle rubbing of the wetted area with the finger-tip quickly reduces the merely dried coating to a sticky paste, whereas the dried and partially sintered coating is unaffected. On an excessively heated and partially or fully matured coating the droplet is not absorbed; a coated foil in this condition will show crazing or cracking of the coated surface under subsequent heating in the final maturing step.

The temperature and time of heating during preliminary sintering must be sufficient to enable the coating to remain intact in the wet rubbing test. At the other extreme the heating must be less than that required to prevent absorption of the water droplet in the absorption test; and the maximum temperature reached during preliminary sintering must be lower than the maturing temperature employed if uniform color and surface appearance are to be obtained in the final product. To allow for unavoidable temperature variations it is ordinarily desirable to maintain a difference of at least 25 F., preferably at least 50 F., between the maximum temperatures reached during sintering and during maturing of the coatings.

The requirement of preliminary partial sintering has the additional advantage of assuring complete removal of water from the slip. It has been found that retained moisture has a tendency to cause pin-holing or blistering or discoloration, and to permit adhesion and offsetting or portions of the coating to the back surface of the next adjacent layer of foil, during firing of the roll. At the same same, complete removal of water but without preliminary partial sintering is ineffective since the coating is then easily cracked and completely or partially removed, by dusting or abrasion or flaking, when the sheet is wound up in roll form for firing.

The application of the aqueous slip to the clean metal foil-may be accomplished in any way desired, eg by spray coating, knife or roll coating, dipping, etc. A presently preferred procedure involves spreading with a coating knife in a continuous operation. The foil is advanced from a stock roll beneath the knife at a desired orifice setting. The coated web passes into a preliminary drier where the moisture content is reduced to about l5-20% by contact with warm air and then into an irradiation chamber where the remainder of the moisture is removed and preleminary partial sintering is accomplished. The web is now ready for winding into rolls or coils and for subsequent firing. In an illustrative operation it is wound on tubular steel or preferably aluminum cores having a diameter of three inches, and to an external diameter of about 7 inches, representing about 100 yards of the web, for firing in an oven. It is preferred to wind the foil with the coated surface toward the core so as to place the coating under compression and avoid any possibility of the formation of hairline cracks which might later appear as blemishes in the matured porcelain surface; but with adequate care the Web may be successfully wound in either direction for further heat treatment.

The procedure makes possible the application of the slip at a relatively rapid rate, thus providing a coating of improved smoothness and uniformity, while still permitting firing for extended periods to attain high chemical resistance and other desired properties.

In one specific example a commercial glass frit is used having the following compositional formula in mol percent: 7.7% Li O, 17.1% Na O, 9.5% K 0, 0.6% SrO, 3.1% CdO, 1.9% ZnO, 4.7% B 0.6% Sb O 1.1% P 0 38.1% SiO, and 15.6% TiO To 1000 parts by weight of frit is added 90 parts of mill additive and 600 parts of water. The mill additive consists of a mixture of 11.5 parts by weight of potassium hydroxide, 31.25 parts of potassium tetraborate, and 57.25 parts of sodium silicate solution having a soda to silica ratio of 113.25 and containing 62.1% of water. The frit is ground in water in a ball mill to 325 mesh, the additive being introduced during the final stages of the milling.

To the resulting slurry is next added silica powder having a nominal particle size of 15 microns and to the extent of 15 parts by weight fo reach 150 parts of the aqueous slurry, i.e. about 97 parts of non-volatiles. The powder is mixed uniformly into the slurry to provide the final coatable slip.

A roll of aluminum foil in dead soft condition as received from the supplier, and which has been heated in an oven at a temperature and for a time sufiicient to drive off all traces of the oily deposits remaining from the rolling operation, is used. The foil is 26 inches in width and has a thickness of two mils (.002 inch). The foil is drawn beneath a coating knife set at a coating orifice of 5 mils where a smooth uniform layer of the slip is applied. The coated foil then passes through a preliminary short warming zone and into a radiant heat chamber at a distance of about three to four inches beneath a series of Glo-Bar rod-like electric heater units spaced about six inches apart and operated at dull red heat, where the water is removed and the dried slip coating is partially sintered. The heater is about six feet in length and the foil travels at a speed of six to eight feet per minute, reaching a temperature of about 900 F. Tension on the web is not greater than about 20 lbs.

Somewhat reduced coating speeds permit drying and sintering at a somewhat lower top temperature of about 850 F. or even about 800 F. The temperature is measured with a thermocouple held very close to, but not in actual contact with, the coated surface.

The coated foil as it comes from the heater is permitted to cool by exposure to room conditions over a distance of about three feet and is then wound on a three-inch diameter core to a final diameter of about six inches, with the coated surface toward the core. Test of the coating as it comes from the sintering chamber show it to be highly water-absorptive but to resist disintegration in the wet rub test. Thermocouples are included at locations next to the core, midway of the winding, and beneath the outer layer, are are connected to a suitable temperature-indicating instrument; and the coil is suspended in a gas-fired furnace which is held between 1025 F. and 1050 F. as indicated on the furnace control instruments. A top temperature of 952 F. is indicated in 3% hours at the outer wrap, and in 4%. hours at the center and inner wraps. Heating is continued for a total of 4 /2 hours. The roll is then permitted to cool and the coated foil is unwound and examined. The porcelain enamel coating is found to have a smooth dense uniform appearance, although containing particles of silica which may be identified under the microscope. It is well bonded to the metal foil; test segments show no loss of coating when the product is wrapped around a half-inch diameter mandrel with the coated surface in either direction. The coating is highly spall-resistant and is resistant to boiling dilute citric acid solution.

In other examples the roll is held at top temperature for much longer times, e.g. up to 16 hours or longer, resulting in porcelain enamel coatings of increased resistance to the acid boil test and with much less evidence of free silica particles when examined under the microscope.

Colored porcelain enamel coatings on aluminus foil are found to have outstanding brilliance and uniformity of appearance when fired in roll form in accordance with this invention. Identical color and surface gloss are obtained throughout the entire roll. Contact between the porcelain coating and the back surface of the next adjacent layer in the roll during the heating process produces no visible blemishes on the porcelain surface. The accurate control of temperature achieved by the slow oven heating process makes possible the duplication of results on successive quantities of coated foil. The process combines prolonged heating with rapid and therefore more accurate and economical coating.

The process is equally useful in the application of multiple coats of enamel. As an illustration, a first or ground coat is applied and partially dried, and a partial top coat is then added, e.g. by spray coating through a stencil or by means of printing rollers, to form a contrasting design. The coated metal foil is then dried and briefly heated for preliminary partial sintering, followed by winding in roll form and heating in an oven at a higher temperature and for a time sufficient to mature the enamel, all as hereinbefore described.

In another example, a commercial glass frit used for enameling steel and identified by the manufacturer, Chicago Vitreous Corp of Cicero, 111., as CV518T, and maturing at about 1400 F., is mixed with mill additive and wet ground to provide a coatable slip. The mill additive consists of a mixture of 2.5 parts by weight of bentonite clay, 1.25 parts of sodium aluminate, and 10 parts of titanium dioxide, based on 1000 parts of the frit. The grinding and mixing procedure is as previously described.

A strip of steel foil identified by the manufacturer as black plate, having a nominal thickness of 1.5 mils (.0015 inch), is washed in an alkaline cleaning solution, rinsed in warm water, immersed for two minutes in a 7% sulfuric acid etching solution at F., rinsed in warm dilute acid, immersed for five minutes in a one percent aqueous solution of nickel sulfate, rinsed in warm Water, passed briefly through a Warm dilute neutralizer bath containing sodium carbonate and borax, and then dried. The dry foil is uniformly spray coated on one surface with the slip as above described in an amount just suflicient to completely obscure the underlying metal Without permitting flowing or sagging of the coating, the foil being held in a vertical position for coating. The strip is passed directly into the radiant heat chamber where the coating is thoroughly dried and pre-sintered by heating to 1075 F. The coating is highly water-absorptive while being highly resistant to disintegration in the wet rub test,

The coated strip is next wound onto a three-inch diameter tubular steel core and with the coated surface inwardly, several turns being applied and held in place against the springiness of the steel foil by an overwrapping 0f uncoated foil which is clamped in place. The coil is placed in a furnace maintained at 1375" F. and held for 15 minutes to mature the coating. It is then removed and allowed to cool; The coated foil is unwound into a flat strip which is uniform in appearance. No offsetting of enamel to the uncoated surface of the foil occurs.

What is claimed is as follows:

1. The method of providing a porcelain coating on thin flexible metal foil, comprising the steps of coating one side of the slip-receptive foil with a fluid porcelain slip, drying the coating and heating to produce partial preliminary sintering of said coating to a wet-rub-resistant but highly water-absorptive state, winding the coated foil into a coil, heating said coil at a uniform temperature, higher than the sintering temperature of said coating, without softening the metal and for a time sufficient to mature the coating, and then uncoiling the coated foil.

2. The method of claim 1, wherein said slip is a fluid slip of a glass-forming composition having an effective maturing temperature below the softening point of said foil and including a glassy frit and free silica, and wherein said uniform temperature is at least about 25 F. higher than said sintering temperature.

3. The method of claim 2, wherein said foil is aluminum foil, said effective maturing temperature is within the approximate range of 900 to 1100 F., and the amount of free silica is sufficient to raise the fusion temperature of said composition to a point above the fusion point of said foil.

4. The method of claim 3, wherein said glassy frit is a silica-containing frit, said coated foil is wound into a coil with said coating in compression, and said coating is matured into a dense acid-resistant state.

References Cited UNITED STATES PATENTS 2,580,406 1/1952 Calton 117 129 X 2,739,085 3/1956 McBride 117-429 X 3,222,219 12/1965 Saunders et a1. 117-231 X 3,383,225 5/1968 Stradley 117 129 X 10 3,389,458 6/1968 Ostrander et a1. 117 129 X FOREIGN PATENTS 547,532 9/1942 Great Britain.

15 ALFRED L. LEAVITT, Primary Examiner J. R. BATTEN, 1a., Assistant Examiner US. Cl. X.R. 20 1171l9.6, 129 

